This invention relates generally to airfoils and in particular to fan blades with multi-material reinforcement.
Fan blades and other structures used in turbine engine applications are susceptible to foreign object impact damage, for example during bird ingestion events (“bird strikes”). Blades made of composite materials such as carbon fiber reinforced epoxy are attractive due to their high overall specific strength, specific stiffness and light weight. However, carbon composites are particularly prone to brittle fracture and delamination during foreign object impacts due to their low ductility. Blade leading edges, trailing edges, and tips are particularly sensitive because of the generally lower thickness in these areas and the well-known susceptibility of laminated composites to free edge delamination.
For best aerodynamic performance, it is desirable to use fan blades which are thin and have a long chord. One problem with such fan blades is that higher strains are encountered in the event of a bird strike as compared to thicker blades having a shorter chord.
It is known to provide impact damage protection for composite fan blades using metallic guards bonded thereto, also referred to as metallic cladding. For example, fan blades are known as having a composite body with metallic cladding extending over the leading edge, the tip, and the trailing edge.
Metallic cladding is generally made of high-density alloys. One problem with their use over extensive areas of an airfoil is that their weight offsets the weight savings from the use of composite material.